To reduce NOx emissions formed during the operation of a diesel engine two methods are employed. A lean NOx trap (LNT) uses Platinum Group Metals (PGM) to adsorb NOx gases on alkali earth oxide materials. The LNT can also generate ammonia (NH3) during rich operation. A selective catalytic reduction device (SCR), which includes a reductant such as NH3, reduces NOx in an oxygenated environment. An external source of NH3, for example a urea solution tank, may be used with a SCR.
One approach is to employ LNT and SCR in series. During primary operation of the engine using lean stratified diesel combustion (lean) the NOx emissions are adsorbed as nitrates by the LNT. However, the engine may be run using rich stratified diesel combustion (rich) periodically to purge the stored nitrates. The LNT may produce NH3 during this phase that can be adsorbed by the SCR to reduce NOx during the next lean run. This combination reduces the need for the external NH3 source.
Some of the problems recognized by the inventors with such a set-up come from running in the rich phase. Running for more than a few seconds in the rich phase produces soot which fills the diesel particulate filter (DPF). High exhaust temperatures, 600-650° C., to regenerate the DPF deteriorates the fuel consumption of the engine. Running for only a few seconds in the rich phase may need high PGM loading on the LNT which increases cost. This also limits the amount of NH3 that can be produced and impacts the performance of the SCR device downstream.
To at least partially address these problems one example includes a method of switching between lean and rich combustion modes where the rich mode is operated using a rich early injection combustion mode (REI), such as premixed charge compression ignition, PCCI. This method may further use lean early injection combustion (LEI) for the lean mode. In this way it is possible to improve the efficiency of the LNT and SCR devices.
In another example, a method of operating a diesel engine system having LNT and SCR aftertreatment devices arranged in series to receive exhaust gas from the engine wherein the method comprises operating the engine in lean and rich combustion modes, switching between the lean combustion mode and the rich combustion mode based upon the operating requirements of the LNT and the SCR to minimise NOx emissions from the engine and using a rich early injection combustion mode when the engine is operating in the rich combustion mode.
In another example, an engine control method includes operating the engine during a first lean stratified combustion mode and operating the engine during a second ammonia generation mode with repeated lean stratified combustion and rich stratified combustion and further operating the engine during a third ammonia generation mode, including repeated lean pre-mixed compression ignition and rich pre-mixed compression ignition. The repeated lean and rich operation in the second and third modes may include switching between lean and rich conditions without stoichiometric operation therebetween. Further, the rich stratified combustion during the second mode may be for a shorter duration than the rich pre-mixed compression ignition combustion of the third mode, even though the lean operation of the second and third modes is comparable (e.g., equal in one example). Further, the duration of the rich combustion in the third mode may be adjusted differently responsive to ammonia storage and/or generation amounts in the exhaust system than the rich combustion in the second mode.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.